The present invention relates to a bottom operated outlet valve for a tank on a vehicle such as a tank on a railway tank car. Such valves are mounted on the underside of the tank and are in normal use opened and closed manually by operation of a valve handle located beneath the railway car. The handle is part of an operator assembly which by means of a mechanical linkage moves a valve stem toward or away from a valve seat. The operator assembly is connected to the underside of a valve body including the valve seat. Since the operator assembly extends down beneath the tank, it is possible that the operator assembly may collide with an immovable object. The prior art has recognized that in this eventuality it is important that the operator assembly purposely shear off the valve body without damaging the valve seat or the valve stem. A compression spring located inside the tank urges the valve stem to its closed position thereby alleviating the possibility of a spill of the tank car contents with possible large economic and environmental consequences.
The prior art outlet valves having bottom mounted operator assemblies and spring loaded valve stems have been difficult to install and maintain. Installation and removal of the operator assembly is complicated by the fact that the operator linkage inside the operator housing must be bolted to the underside of the valve stem while the quite heavy operator housing is held near the valve body, yet far enough away to reach inside between the valve body and operator housing. The operator housing can not be bolted to the valve body until after this precarious linkage to stem bolting procedure is accomplished. When the operator assembly is to be removed for repair or replacement of the valve, this difficult procedure oftentimes must be accomplished in reverse.
In the event that the valve stem seal or seat should become worn or deteriorated, the valve will leak since the linkage always holds the stem in a set position relative to the seat. The prior valve provides no means for the valve stem to compensate for this problem. Further, the design of the valve body of the prior art has made the replacement of valve stem seals relatively difficult and somewhat dangerous to accomplish, in that during removal of the valve stem the entire force of the valve stem compression spring must be released all at once. During reassembly the spring must be fully compressed before the spring can be mechanically locked in place. This causes great difficulty in locking the spring in place.